Abstract Perovskite solar cells (PSCs) have attracted much attention since the first report. Flexible PSCs are one of the important development orientations due to its light weight. However, a low temperature process is essential to fabricate the flexible PSCs due to the deformation of the plastic substrate. Here, the pre-crystallization SnO₂ colloids have been used to deposit the electron transport layer of flexible PSCs. The photovoltaic performance of flexible PSCs has been optimized through controlling the annealing temperature and SnO₂ thickness. A moderate annealing temperature reduces the deformation of flexible substrates and protects the conductive layer. A SnO₂ film with a suitable thickness not only blocks the direct contact between perovskite and electrode, but also transports the electron from perovskite to anode efficiently. As a result, the power conversion efficiency of the champion PSCs has been improved to 11.61 %.